Various conversion coatings on metal surfaces have been suggested in the prior art for the purpose of forming a coating which protects the metal against corrosion and also serves as a base for improving the adhesion of subsequently applied siccative organic finishes. Such conversion coatings are applied by treatment of the surfaces with solutions of various chemicals which react with the surface to form the desired coating. Among the commonly used conversion coating compositions are aqueous phosphate and chromate solutions. Among the simplest of the phosphate compositions are the so-called iron phosphates which comprise, for example, solutions of alkali metal phosphates, and which react with the iron on the metal surface to form an iron phosphate coating.
It also has long been known that surfaces of zinc and zinc-based alloys can be protected against corrosion by treatment with an acid solution containing hexavalent chromium. It was suggested that the attack of the solution on the surface was facilitated if the solution initially contained a small amount of trivalent chromium, and it has been proposed to introduce this trivalent chromium by adding a compound of trivalent chromium, or preferably by adding a small amount of a suitable reducing agent. As the solutions are used, more trivalent chromium is formed by reduction of hexavalent chromium at the zinc surfaces so that the concentration of trivalent chromium progressively increases and the solution eventually has to be discarded when the quality of the coating is affected by the deterioration of the solution. Examples of patents which describe solutions containing mixtures of trivalent chromium and hexavalent chromium include U.S. Pat. Nos. 3,880,772; 3,795,549; 3,553,034; 3,404,046; 3,090,710; 2,911,332; and 2,902,392.
The treatment of zinc surfaces with solutions wherein chromium is entirely in a trivalent state is disclosed in, for example, U.S. Pat. Nos. 3,932,198; 3,647,569; 3,501,352; and 2,559,878. Trivalent chromium solutions also are disclosed in British Patent 1,461,244.
One disadvantage of hexavalent chromium type solutions is in the area of waste disposal. Emphasis on water pollution problems has drawn attention to the fact that chromates are serious pollutants. In order to satisfy water quality standards, it frequently is necessary to subject the waste water to a multi-stage purification sequence in order to remove chromates from the effluents. Typical steps in the sequence include the reduction of any hexavalent chromium to trivalent chromium and precipitation with, for example, lime. This precipitation results in a reduction in the chromate content of the effluent water but the process is quite expensive, and the precipitate creates a disposal problem.
Another problem which has been observed with chromate finishes which have been described previously is the unacceptable adhesion characteristics when certain paints have been applied over the chromate coatings, particularly on exposure to salt spray.
Chromate coating solutions containing trivalent chromium as substantially the only chromium ion present, fluoride ion, an acid other than nitric acid and an oxidizing agent such as an inorganic halate or peroxide have been described in U.S. Pat. No. 4,171,231. Such solutions deposit desirable light to clear blue chromate finishes.
U.S. Pat. No. 4,263,059 describes aqueous acidic chromate coating solutions for treating zinc, zinc alloy or cadmium surfaces which comprises trivalent chromium as substantially the only chromium ion present, fluoride ion and an acid wherein the coating solution is prepared by mixing a green trivalent chromium ion solution with a blue trivalent chromium ion solution having a pH of less than 1. The blue trivalent chromium solution can be prepared by dissolving a source of hexavalent chromium in water and adding a reducing agent to reduce the hexavalent chromium to trivalent chromium and thereafter adding fluoride ion and an acid to reduce the pH to less than 1. The acids may be organic acids such as acetic acid or inorganic acids such as nitric acid, sulfuric acid, hydrochloric acid, sulfamic acid and phosphoric acid.
U.S. Pat. No. 4,026,728 describes coatings for steel sheet including zinc plated steel, chromium-plated steel, aluminum-plated steel, etc. with a solution containing at least one compound selected from the group consisting of chromic acid, phosphoric acid, salts of chromium, molybdenum, silicon, cobalt, manganese, copper, nickel, aluminum and titanium, and thereafter contacting the treated steel with a solution containing at least one organic silicon compound.
U.S. Pat. Nos. 2,559,878; 3,647,569; and 3,932,198 describe solutions for coating metal surfaces which comprise trivalent chromium and nitric acid. In the '198 patent, the solutions also must contain one or more cations selected from the group consisting of manganese, bismuth, antimony, tin, zinc or molybdenum.